The present invention relates to a spectrometer, and, in particular, to a spectrometer that analyzes the spectrum of light emanating from one or more targets within an image.
U.S. Pat. No. 5,479,258 issued Dec. 26, 1995, to Hinnrichs discloses a spectrometer comprising a diffractive lens, a planar array of photodetector elements (pixels), means for changing the distance between the photodetector array and the diffractive lens along the optical axis, and a signal processor. If either the photodetector array or the lens is moved along the optical axis, different wavelengths of light come into and out of focus on the photodetector array, thereby generating sequential images that correspond to different wavelengths. The spectral composition of a target can be determined from the output of the photodetector array as a function of the position of the diffractive lens relative to the array. Targets have a fine spectral signature that is easily differentiated from the background (non-target) radiation, which generally appears as broad-spectrum white light. Processing to eliminate this background radiation may enhance the sequential images from the array of photodetector elements. Each target's spectrum is built up from sequential images and can be compared to a library of known target signatures.
The basis for Hinnrichs' design is the inherent chromatic aberration of a diffractive lens. The principal shortcoming of such a design is that the magnification is different for each wavelength in the recorded spectra. This shortcoming creates problems in image registration, provides inaccurate relative spectral signal strength, and yields an image space defined by shift variant transfer/imaging theory, thereby requiring complex and computationally intensive calculations.
Problems can arise with this prior-art technique. Resampling the detected image cube requires averaging a non-integral number of pixels, causing edges to blur and pixels to register inaccurately.
Thus there exists a need for a spectrometer with a more accurate optical method of obtaining an image cube with constant magnification.